The field of optical telecommunications has experienced phenomenal growth over the past several years, fueled in large part by the development and deployment of erbium-doped fiber amplifiers ("EDFAs").
Prior to optical amplifiers, fiber optic communication systems required electronic repeaters to amplify signals and offset the losses associated with long distance optical transmission. These electronic devices converted the transmitted optical signal into the electrical domain, amplified and reshaped the electrical signal, and converted the electrical signal back into an optical signal, and onto the next leg of the fiber system.
Electronic detectors are incapable of discriminating between different wavelengths, therefore, multi-wavelength systems require an independent repeater for each wavelength in the system, as well as the necessary filtering components to isolate each of the wavelengths into their respective repeaters. FIG. 1a is a representative bock diagram of a 16 channel repeater system 10. The large amount of equipment needed made multi-wavelength communications systems using repeaters prohibitively expensive.
The deployment of the EDFA, however, changed the topology of fiber-optic communication networks. Because the optical amplifier was capable of amplifying multiple wavelengths independently in a single unit, a multi-wavelength system could use a single optical amplifier. The 16 channel repeater system of FIG. 1a requires a wavelength de-multiplexer 12 to break the 16 channels out onto individual fibers 14, 16 repeaters 18, and a wavelength multiplexer 16 to recombine the 16 individual wavelengths. All of this was replaced by the single EDFA 20 shown in FIG. 1b.
This ability to add multiple channels to a single fiber with minimal additional amplifier costs created an explosion in the bandwidth of optical fiber communication systems. Deployed systems went from single channel sub-gigabit per second operation to 128 channel, multi-gigabit per second per channel operation.
The continuing growth in the bandwidth of optical communication systems has also led to a new demand for more complex functionality in optical networks. To satisfy this demand, manufacturers are desirous of devices in increasingly smaller and more compact packages, while at the same time integrating multiple functions into a single device. In sharp contrast to the ever-shrinking size of many optical network components, the EDFA faces a hard limit because of the fixed value of the minimum bend radius of the fiber. Bend losses are proportional to the bend radius, and hence the radius must be kept large. A nominal radius for such fiber is about 3.75 cm, resulting in a coil diameter of about 7.5 cm (about 3 inches) which in turn results in an minimum package planar dimension of at least 31/2.times.31/2 inches. Consequently, though many components such as the isolators, 980/1550 nm multiplexers, and monitor taps and photodiodes can be integrated into the package, the footprint remains constrained by the minimum diameter of the erbium-doped fiber coil. Nominal package dimensions are now about 6.times.6 inches.
Semiconductor optical amplifiers (SOAs) are also available, and rely on electrical (rather than optical) pump sources for amplification. However, their performance characteristics are known to be deficient in many areas, compared to EDFAs.
The demand for more compact devices with greater integrated functionality is accompanied by the need to reduce the cost of these devices. As optical system designers increase the complexity of their networks, the number of optoelectronic components required grows substantially. For the very same reasons that multi-wavelength systems with electronic repeaters were prohibitively expensive, more complex networks begin to face the same cost issues. Thus, not only is "real estate" within the system at a premium requiring smaller, integrated devices, but cost also plays a significant role requiring less expensive components as well.
The EDFA was the critical enabling technology that gave rise to the recent boom in optical communications known as WDM. As communications systems continue to grow and expand, system designers find themselves in need of new devices, amplifiers in particular, that are smaller and less expensive than their current counterparts. A compact, low cost optical amplifier with the ability to incorporate additional devices for added functionality is therefore required.